Depending on the particular type of turkey you buy, ground sirloin may actually be healthier. It has fewer calories, nearly half the fat, and less cholesterol. That's because regular old ground turkey contains dark meat, which is fattier. If you do go with turkey, make sure it's turkey breast or at least turkey white meat. Otherwise just go for the ground sirloin (96/4).
FWIW, while I wouldn't make a 100% ground sirloin burger, I use it regularly in dishes that are slow cooked and/or contain a lot of spices, like chili and tacos.